NEW PALEONTOLOGICAL DATA FOR GREAT VALLEY GROUP SEDIMENTS IN THE GUENOC RANCH AREA, LAKE COUNTY, CALIFORNIA

New geologic mapping on the Guenoc Ranch in Lake and Napa Counties has delineated a distinctive lithofacies of carbonate rocks within what has been previously mapped as Lower Cretaceous Great Valley Group (GVG) marine sediments. These carbonate rocks have subsequently been identified as cold methane seep deposits (Hepper 2004). A radiolarian assemblage where Psuedoaulophacos lenticulatus (White) is the only species identified to date indicates a lower Coniacian to upper Campanian age for these rocks (Kariminia, written commun, 2003). An ammonite from the same sample locality of genus Didymoceras or Nostoceras, of Campanian to Maasstrician age (Elder, oral commun, 2003 and Hepper, 2004) was found. The seep carbonate rocks yielded bivalves Lucinidae and a single Solemyidae, an Inoceramidae of possible Campanian to Maasstrician age and Vestimentiferans (Hepper, 2004).

The GVG feldspathic wacke that hosts the seep carbonates is fault bounded on the northeast by Jurassic basal Coast Range Ophiolite. Eighteen miles to the northwest, along the structural trend, GVG marine sediments hosting similar carbonate rocks yielded the same radiolarian fauna, Psuedoaulophacos lenticulatus (R. J. McLaughlin, written commun, 2005), suggesting a stratigraphic and structural link of this unique cold seep assemblage to structurally imbricated Mesozoic rocks of the Clear Lake area.